These are the tough ones as you know. Of course they predate your work on the pottery production in Southold, down the road from where George Washington spent 3 nights waiting for the ship to Boston, in Greenport, after the "French and Indian War" on advisement of a doctor, to see what's going on there, according to the rock there commemorating that widow's place whose name escapes me, there with Edward Johanneman and then later the Institute for American Studies at Cooperstown, NY, hand auguring for pottery samples in the front yard of exotic wood constructed sea captain's house on the state register you may know, for an exhibit at the Suffolk County History Museum of Long Island potteries. Those perhaps later "wares" are all to my knowledge, redwares, from the clay there and became the largest employer, some in barter and exchange, on the east end of Long Island, reported to be as many as 25 people working there! 

There is this story I will relate about Huntington, NY also quite old called, "Ashford" during the time of Oliver Cromwell, as a few places or toponyms changed then, as seen on a map in their library, I'm not sure in protest or in the "new world order" returned perhaps on the Restoration of Charles II. Huntington, as you may know, had the famous "Brown Brothers" pottery works, which produced quite a bit of saltglazed stonewares along the lines you've described, that is blue decorated jugs and larger containers for storage, etc. The story part goes that before them, and apparently after them the fine clay was sold off, there were other potters, yet to be archaeologically documented, perhaps under a current marina storage yard where they were thought to be, as you have found the Greenport Pottery to be under fill and along the shore. This fine clay was rumored to have been used in the production of fine earthenware vessels, yet unsubstantiated, and that particular clay sold, depleted, or interrupted, as was later described in the State of Ohio perhaps, as late as the 1930s, when the Ph.D. thesis I read was accepted, in regards to the importation of clay from primarily England, to wit, American clay would not fire as well. So perhaps you may actually be looking at a local Long Island fine saltglaze from a little further west, from Huntington, predating its better known "Brown Brothers" pottery. 

There are some interesting clay banks just outside the Garvies Point Museum I've sampled and was surprised of their firing, without temper, in my Mom's oven, to be quite sturdy and strong for even the low temperature produced in a conventional oven. It would be interesting to contact some of the other people who have worked in this very problem who have taken small drilled samples from different wares to determine their mineral signatures and tried to determine their origin, particularly redwares as I recall found in early contexts associated with New Amsterdam, i.e., NYC. Perhaps Professor Gilbert at Fordham University may know of them and others, as I recall he was the earliest I encountered in that line research and has an extensive ongoing collection of bricks from the various brickyards and sites in New York and New Jersey. 

George Myers